The Environmental Protection Agency (EPA) has estimated that the annual generation of hazardous wastes in the United States to be in excess of seventy billion gallons. Hazardous wastes include organic materials, such as polychlorinated biphenyls, pesticides, herbicides, municipal garbage, hospital wastes including pathogens, paints, inks and contaminated solvents, black liquor and explosives. Examples of inorganic wastes which are a serious environmental problem include oxides of iron, zinc, copper, lead, magnesium, aluminum, chromium and cadmium, various powdered metal manufacturing residues and metal-containing sludges.
Further, the EPA classified as toxic, ignitable, corrosive or dangerously reactive many common waste materials, such as paint sludge from appliance factories, dregs of chrome and nickel from metal plating shops, spent raw materials for varnish, carpets and detergents at chemical plants, solvents from dry-cleaned clothes and degreased microchips from computers, mercury from exhausted watch batteries, butane residue from disposable cigarette lighters and lye from cans of oven cleaners.
Landfills are becoming less available as a means of disposing of such wastes. In the absence of suitable landfills, hazardous wastes must be transformed to benign and preferably useful substances. There has been tremendous investment in development of alternative methods of treating hazardous wastes. Various types of reactors which have been employed for decomposition of hazardous wastes include for example, rotary kiln, liquid injection, multiple hearth, multiple chamber, fluidized bed, molten salt and high efficiency boilers.
A common reactor for pyrolytic and combustive destruction of organic wastes is the rotary kiln. Pyrolysis in rotary kilns typically is initiated by a high temperature flame. The reactive species generated by the flame are then oxidized by contact with oxygen. However, because the waste material is often highly viscous, it is generally difficult to contact reactive species with oxygen for combustion of the reactive species. The rate of destruction of the waste, therefore, can be impeded. Moreover, heat released by combustion occurs away from the flame tip as reacting materials are fed through the rotary kiln, thereby limiting the heat which can be utilized for the initial pyrolysis step.
Titus et al., U.S. Pat. No. 3,812,620, disclose a molten pool of glass and miscellaneous metals formed during incineration of "heterogenous waste materials," such as municipal garbage. Various organics in the waste materials are pyrolyzed in the molten pool at temperatures of up to 10,000.degree. F. However, the decomposed materials are not contacted with oxygen introduced into the molten pool.
Rummel, U.S. Pat. Nos. 2,647,045, 2,923,260 and 3,953,445, disclose molten slag bath formed from reduction of iron ore or from the "non-combustible residues of coal products." Finely divided coal is injected into the bath and separate addition of air is conducted with "an endothermic reactant," i.e., water. The process is preferably conducted in two separate endothermic and exothermic zones.
Rassor et al., U.S. Pat. Nos. 4,187,672 and 4,244,180 disclose molten iron employed as a solvent for carbon generated by thorough topside introduction of coal. The coal is then partially oxidized by iron oxide and by introduction of oxygen from above. However, carbonization and oxidation are conducted in distinct carbonization and oxidation chambers.
Molten salt processes are also known, and are typified by the reaction of alkali metal carbonates in a basic, or caustic, molten bath into which oxygen and wastes are injected. However, such baths generally rely principally upon thermal distribution rather than chemical reaction to degrade waste materials. See, for example, U.S. Pat. Nos. 4,447,262, 4,246,255 and 4,017,271.
Large scale destruction of PCBs can be done in large scale high efficiency boilers by their addition to conventional fuel in amounts up to 5%. See "Destruction of High Concentration PCBs in a Utility Boiler," by Siedhoff, Zale and Morris, Proceedings of the 1983 PCB Seminar, Electric Power Research Institute. However, the long-term corrosion and other effects on such high efficiency boilers is largely unknown.
Oxidation of wastes by reaction of organic materials and oxygen in the presence of water at high temperature and pressure is another alternative method of disposal. See, Wilhelmi, A. R. and Knoop, P. V., "Wet Air Oxidation--An Alternative to Incineration," Chemical Engineering Progress, 75:46-52 (1979). However, inorganic salts, char and metal-containing solids can accumulate at the walls of apparatus employed for such methods, thus necessitating removal of accumulated deposits, causing corrosion and limiting the operating capacity and useful life of such apparatus.
Bach et al , U.S. Pat. Nos. 4,574,714 and 4,602,574 disclose a unitary process wherein organic and inorganic wastes are injected conjointly with oxygen into a molten metal bath. The molten metal bath is comprised of metals and oxides of metals having free energies of oxidation under the conditions of the molten metal bath which are greater than the free energy of oxidation of carbon to carbon monoxide. The waste materials are pyrolytically decomposed and oxidized in the presence of the molten metal bath to form carbon monoxide. The carbon monoxide can be oxidized by contact with oxygen gas introduced into the molten metal bath.